In recent years, complementary metal-oxide semiconductor (CMOS) imagers have been increasingly widely used in digital still cameras, camcorders, surveillance cameras, and the like and also the market thereof has increasingly expanded. In such a CMOS imager, each pixel circuit converts incident light into electrons using a photodiode, accumulates the electrons for a fixed period, and then outputs a signal reflecting the amount of accumulated electric charge thereof to an analog-to-digital (AD) converter built in a chip. The AD converter digitizes the signal, and then outputs the digitized signal to the following stage. In the CMOS imager, pixels are disposed in a matrix shape for imaging.
A general pixel circuit is provided with a photodiode, a transfer transistor, a reset transistor, an amplifier transistor, a floating diffusion layer, a selection transistor, and the like. Photons incident on a silicon substrate of the pixel circuit generate pairs of electrons and holes, and then the photodiode accumulates the electrons in a node on the side of cathode of the photodiode. The electrons are transferred to the floating diffusion layer by switching on the transfer transistor at a predetermined timing to drive the gate of the amplifier transistor. Thus, signal charge turns into signals of vertical signal lines to be read through the selection transistor.
The amplifier transistor and the vertical signal line are connected with a constant current circuit. The constant current circuit configures a source follower. In addition, a signal of a charge accumulation layer is somewhat attenuated with a gain of slightly less than 1, and output to the vertical signal line.
In addition, the reset transistor is switched on at the same time as the transfer transistor, takes out the electrons accumulated in the photodiode to a power supply, and thereby resets the pixel circuit to a dark state before the accumulation, i.e., a state of no incident light. As the power supply used for the reset and source follower, for example, 3 V is supplied.
Such a CMOS imager has had decreased parasitic capacitance inside pixel circuits due to miniaturization of the recent years, which improves conversion efficiency and sensitivity. Here, a detection node that is an amplifier input of the above-described source follower is formed with a drain diffusion layer of a transfer transistor, a gate electrode of the amplifier transistor, and wiring which connects both. If parasitic capacitance decreases due to miniaturization, a potential of the detection node for signal charge generated by the photodiode significantly fluctuates accordingly, a pixel output to signal lines via the source follower also increases accordingly, and thus sensitivity improves.
Furthermore, CMOS imagers have exhibited improved crystal quality of substrates, and a reduction of dark currents and low noise of amplifier transistors have progressed. In other words, the signal-to-noise (S/N) ratios of signals have remarkably improved, and according to such trends, a device that uses this as an optical detector that deals with ultra-weak light such as fluorescence has been proposed (for example, see Patent Literature 1).
As one of expected applications, a radiation counting device combined with a scintillator is exemplified. Radiation counting (photon counting), in which a dose of radiations incident on a detector is counted while energy of the respective radiations is distinguished in units of incident photons, has been recently applied to various fields, such as dosimeters and gamma cameras. As such detectors, scintillators and photomultiplier tubes have been mainly used in recent years. If such a detector is replaced with a CMOS imager, a dramatic reduction in size and weight of the device can be realized. Further, the radiation counting method can also be introduced to X-ray imaging and computed tomography (CT) imaging that require high spatial resolution, and a drastic reduction in an exposure dose can be realized.